Since an amount of consumption of energy such as electric energy always fluctuates in accordance with living activities of human being, a balance of demand and supply of energy is bad. When nuclear energy which is difficult in adjusting output and natural energy which is irregular are combined in order to compensate for energy, it is apprehended that an amount of power consumption cannot be adjusted, resulting in an increase in surplus power. If use of renewable energy increases in the future, there is a limit in adjusting output by thermal energy.
Though it is considered to transmit this surplus power to a region where electric power is in short, problems of energy loss, a high cost, and so on due to power transmission occur. As solutions for such problems, pumping of water by pumped storage power generation and power storage by a storage battery can be cited. However, there are problems that land suitable for placing a dam capable of pumped storage power generation is limited and that the storage battery is high in cost. As described above, the electric energy has problems related to power storage and power transmission.
Meanwhile, as renewable energy which is storable and transmittable, an artificial photosynthesis system is suggested. In view of an energy problem and an environmental problem, a technology to efficiently reduce CO2 by light energy as if by a plant is required. The plant uses a system called a Z scheme in which two stages of excitation occur by light energy. In other words, the plant obtains an electron from water (H2O) by light energy and reduces carbon dioxide (CO2) by using this electron, to thereby synthesize cellulose and saccharides. As a device to carry out artificial photosynthesis, a photoelectrochemical reaction device reducing (decomposing) CO2 by light energy is being developed. In order to convert chemical substances obtained by those artificial photosynthesis systems into chemical products, a new device is required to be introduced, but stable supply of materials is difficult due to the fact that the material is unstable natural energy, so that a stable operation of the device is impossible, which results in a bad efficiency. Thus, system design is necessary where a reactor converting carbon monoxide or the like into methanol or the like in a manner to follow unstable natural energy is operated stably and where a electrochemical reaction device is efficiently used to heighten a conversion efficiency.
Natural energy is unstable and it is impossible to always supply stable energy. When chemical energy is generated from such natural energy and chemical products derived therefrom as materials are produced, there are problems that an introduction cost of a new device is necessary and that a stable device operation is unavailable due to instability of energy supply.